Persons skilled in the art of tapping metal know that it is desirable and advantageous to separate the molten metal bath from the slag floating on the surface of the bath to obtain clean, slag-free metals. To accomplish this task, various apparatus and methods for impeding slag from entering the tap hole of a metallurgical vessel during the tapping process have been proposed.
One early 19th century smelting and refining furnace, described in U.S. Pat. No. 589,210, purports to remove slag from the surface of the molten metal bath by constantly blowing a stream of air across the surface of the bath.
The furnace consists of two successive adjacent chambers, the first chamber being lightly sloped towards the second chamber. The second chamber is positioned adjacent the first chamber at a lower elevation. A wall having a discharge opening separates the chambers. Or is smelted within the first chamber. The matte which results from the smelting sinks to the bottom of the first chamber and the slag floats atop the matte. When the volume of matte within the first chamber rises to the level of the discharge opening, the stream of air forces the surface slag through the discharge opening into the second chamber. The molten metal or matte is then drawn off through a bottom tap hole. Unfortunately, this apparatus does not prevent the remaining slag from being drawn into the molten metal stream.
Another early method of preventing slag from flowing through the tap hole of an Open Hearth type furnace is shown in U.S. Pat. No. 2,246,144. Here a raft is proposed which floats on the bath above a tap hole located in the bottom side of the furnace. As the molten metal drains through the tap hole of the furnace, a vortex forms in the bath. The raft is purportedly positioned over the vortex and allowed to descend with the bath as the molten metal pours through the tap hole, thereby impeding the entrainment of the slag into the vortex. The raft is adjustably positioned on the bath surface by a boom which extends through an opening in the vessel. The boom is adjustably secured to a movable cart on the outside of the vessel. The design of many modern vessels makes the use of this apparatus and method unfeasible.
Other apparatus have been suggested to impede slag from mixing with the flow of molten metal in a top pour ladle by directing hot gas streams onto the bath surface to prevent slag from entering the ladle spout. In a top pour ladle, the ladle is tilted until the molten metal pours over the lip or spout, much like the common soup ladle. U.S. Pat. No. 2,828,516 shows a spout design having a dam-like partition which holds back the slag atop the pool of molten metal directly adjacent to the spout. In addition, hot gas is directed onto the surface of the pool to force the slag away from the spout to allow the bath to flow cleanly from the spout. After a majority of the bath is poured from the ladle, the ladle is tilted in the opposite direction so that the remaining metal and slag can be poured through a slag outlet. Top pour ladles, however, are rarely used in contemporary mill operations such as continuous cast operations where bottom pour vessels are used in the casting process to fill tundishes which in turn drain into the continuous casting mold. One reason for this is the inherent limitation of the extent to which such a vessel may be tilted. Another reason is that contemporary continuous cast operations generally utilize bottom pour vessels.
Floating ceramic bodies and stoppers which plug the tap hole once the bath volume within the vessel has been substantially tapped have also been proposed (See U.S. Pat. Nos. 4,431,169; 4,462,574; 4,706,944). The floating bodies, which weigh more than the slag but less than molten metal, are placed within a vessel. The stoppers are generally attached to the end of a rod which is positioned above the tap hole of the vessel. When the bath has substantially drained from the ladle, the bodies and/or stoppers are used to plug the tap hole to prevent slag from flowing through the tap hole. These methods, however, do not prevent slag from being entrained in the vortex created by the flow of metal through the tap hole. Moreover, the bodies and stoppers are generally unreliable and are consumed during a heat due to the extreme heat of the bath. The bodies and stoppers are also expensive to replace.
Still another proposed apparatus and method to separate molten metal from slag during the tapping process includes a least one gas permeable refractory element located in the vessel adjacent the tap hole (See U.S. Pat. Nos. 4,079,918; 4,360,190). A gas jet is introduced into the bath through the element in the area where the vortex would be formed. The gas intersects the vortex area, purportedly restraining the slag from being entrained in the tapping process.
Lastly, another method and apparatus purports to promote slag free tapping of metal from a Basic Oxygen Furnace (BOF). Generally, BOFs' have a tap hole in an upper side wall section. Tapping is accomplished by tipping the vessel so that the molten metal flows through the tap hole. The apparatus includes a plurality of refractory devices arranged around the tap hole. When the BOF attitude is modified for tapping, i.e. tipped, jets of gas are discharged through the refractory devices into the bath. The gas jets purportedly move the slag away from the area of the molten metal above the tap hole so that the slag free metal is tapped. Unfortunately, the refractory devices are fixed and cannot be adjusted to maximize each jet's potential for impeding slag from entering into the tap hole.
None of the methods heretofore proposed have proved to be completely successful in eliminating the inclusion of slag in the molten metal during tapping, particularly in vessels which have a moving level of molten metal such as those commonly used in contemporary melting operations.